1. Field of Invention
This invention relates to processes for curing chloroprene/sulfur copolymers which do not form nitrosamines.
2. Background Art
Conventional chloroprene/sulfur-copolymers are in general made by radical induced emulsion polymerization of chloroprene (2-chloro-1,3-butadiene) in the presence of elementary-sulfur and, if required for certain applications, of a copolymerizable diene such as 2,3 dichloro-1,3-butadiene or an acrylic derivative. The highly viscous polymers so formed are plasticized by treating the latex with a peptization system. Such systems are usually based on thiurams (1) and/or dithiocarbamates (2), which are known to form carcinogenic nitrosamines (3) during processing and storing of the rubber, the compounds and the final article. ##STR1##
U.S. Pat. No. 3,984,609 to Branlard et al. describes the peptization of chloroprene/sulfur copolymers, made in the presence of a modifying agent such as iodoform or a dialkylxanthogen disulfide, by adding to alkaline latexes of such copolymers a modifying agent which is a thiol of the formula R-SH wherein R includes, amongst other groups, an arylthiazole so as to give mercaptobenzothiazole as the modifying agent. The curing systems used with these copolymers use a nitrosamine forming accelerator, i.e., 2-mercaptoimidazoline.
Nitrosamine formation with the above systems can be avoided by curing the copolymers with metal oxides alone or omission of 2-mercaptoimidazoline as an accelerator. However, in these cases there is an insufficient cure.
A goal in the rubber industry is to provide the outstanding processability of conventional chloroprene/sulfur copolymers, which use thiurams as peptizing agents, while at the same time avoiding the formation of nitrosamines. The processability of such copolymers is characterized by (1) further peptizability during compounding and curing, (2) reasonably long storage stability for safe handling and low scorch, and (3) high rate and final state of cure when cured by metal oxides only. The present invention meets this goal.